With the advent of large scale farming on tracts of land measured in thousand of acres instead of tens of acres, and the development of farm machinery of massive size to handle such acreage, problems arose which were either non-existant or much more simply overcome by the small farm farmer. For example, in making a single pass of a colorless pesticide spray or fertilizer spray, the small farm farmer usually had no difficulty in determining the dividing line between the sprayed portion and the unsprayed portion and could therefore direct his equipment without much difficulty. However, with large scale machinery where a single pass may be as much as from 50 to 70 feet in width, it becomes a very serious problem to determine where to make the next pass. Too much pesticide or fertilizer or too little of either can cause severe damage to a crop, particularly, if the area of either overlap or skipping is of an order of magnitude of ten feet over a distance of 1 or 2 miles. Too much pesticide, i.e., a double dose for such a width can kill from one to three acres of crop through overdose or permit the crop to fail in an equivalent area because of failure to apply any dose whatever.
Liquid used for soil treatment in applications of fertilizers, herbicides, pesticides and other soil treatments is often applied to the ground surface by spraying from an elongated boom moved over the field surface behind a tractor. The liquid is fed to the boom from a container either carried on the spraying device or otherwise attached to the tractor. The boom is typically mounted in a transverse orientation to the direction of movement to the tractor in order to produce a wide application pattern. The overall length of the boom enables application of the liquid in a minimum number of passes or swaths across the field. The difficulty, however, is in knowing exactly where the boundary of the previous liquid application swath lies since it is desirable to avoid overlapping applied liquid, or leaving areas or gaps of ground surface untreated.
Various forms of marking devices have been developed as suggested solutions to this problem. These marking devices include ones which physically mark the ground by means of a cutting disk, as disclosed by U.S. Pat. Nos. 3,903,970, 3,072,200, 3,766,987, and 3,524,508; use liquid marking mediums dispensed by the treatment apparatus, as disclosed in U.S. Pat. Nos. 2,199,421, and 3,019,470; use powdered marking mediums, as disclosed by U.S. Pat Nos. 3,198,383, 3,443,727, and 3,531,024; and use a foam marking medium, as disclosed by U.S. Pat. No. 3,481,545.
Of the above references, none provide a simple yet adequate solution to the problem of marking the boundaries of treated soil or ground surface. Devices which physically mark the ground surface by means of cutting disk are not feasible in previously planted fields since the marking device will destroy a portion of the crop. Devices applying liquid markers often employ dyes, paint, or other materials that are not easily distinguishable from a distance; often such materials are harmful to the soil and not readily available to the user. Powder markers have the drawback of potential permanence; for example, a lime deposit used as a marker will remain on the ground surface until plowed under and this could cause confusion if subsequent applications of liquid need be applied as the previous markers could be confused with present markers. While foam markers overcome some of the disadvantages of these marking devices, all of the marking devices share the limitation that their markers are not easily distinguishable from a distance or in less than ideal lighting conditions, including dusk and darkness.